I appreciate your attempt to help, Charlie. But why do you think I'd be able to complete those steps, when the errors I'm getting, as I mentioned in my first post, keep me from creating a symlink, the folder icon in your first step? Is there something I'm overlooking?

I would love nothing more than to finally get past those errors. But I don't see how I can now, when the permission setting for the external drive you mentioned is already checked, and is not the solution I hoped it would be. Can you still find a way to help me, or have I run out of options, and have no choice but to give up in frustration?

the errors I'm getting, as I mentioned in my first post, keep me from creating a symlink

Sorry, I don't see anything in any of your posts that specifically says the symlink was not created, as opposed to other possible problems. Are you saying that the /Library/Application Support directory does not contain a folder icon named BOINC Data with the little curved "alias" arrow in the bottom left corner of the icon?
Here's another way to check for the symlink. In Terminal, enter this:

cd "/Library/Application Support"
ls -al 

If the Symlink exists, you should see:
BOINC Data -> /Volumes/Mercury Elite Pro/other/BOINC Data

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Charlie Fenton
BOINC / SETI@home Macintosh & Windows Programmer